This invention relates to the reduction of oxidation of aluminium and aluminium alloys.
Diffusion bonding is an extremely useful technique for joining metal components, particularly in the aircraft industry, and involves the pressing together of the components when heated so that their atoms interdiffuse at the interface, thus forming a metal-to-metal bond between the components. Diffusion bonding can be combined with the process of superplastic forming if certain types of metal are used that have a specific uniform grain structure (for example titanium or aluminium) thus enabling the manufacture of multi-sheet components of complex structure. Superplastic forming is a technique in which a heated metal is subjected to slow deformation during which the metal stretches (up to several hundred percent) without fracture or significant necking. In order to manufacture these components it is often a requirement that the metals are not bonded at all in their contacting areas. Therefore, bond inhibitors (commonly known as stop-off or stopping-off materials) are applied to selected areas by, for example, a silk screen printing process.
Titanium is a particularly suitable material for use in such processes because it will absorb its own oxide layer at high temperature in an inert atmosphere to provide an oxide-free surface necessary for satisfactory diffusion bonding.
However, the physical properties of aluminium and many of its alloys, i.e. low density and high strength, make it an ideal material for use in aircraft components and the like. Such materials, if left exposed to the atmosphere for even a short time, acquire an extremely tenacious surface oxide layer that inhibits or prevents satisfactory diffusion bonding because it is not absorbed at the required temperatures. This oxide layer forms quickly even at very low partial pressures of oxygen, and therefore it is extremely difficult to remove and maintain the surface totally oxide free prior to and during diffusion bonding.
For the sake of brevity, aluminium and its relevant alloys are hereinafter referred to as simply "aluminium", and any references thereto should be construed accordingly.
Several prior proposals for overcoming the problem of the oxide layer and enabling the successful diffusion bonding of aluminium have been made. For example, in our earlier European patent number EP-A-0350220, we described a method of removing the oxide layer of an aluminium component by grit blasting it and then subjecting it to a chemical treatment. Although this technique greatly reduces the problem of the oxide layer, it has been found that, despite the fact that the oxide is removed before diffusion bonding takes place, an extremely thin oxide layer is still present in the component in areas where bonding has occurred. This is thought to occur due to trace oxygen present during the bonding process, and the result is a fairly poor diffusion bond which has a low peel strength.
In another of our European patent applications, number EP-A-0398760, we describe a method of encapsulating the de-oxidised aluminium components immediately after cleaning to prevent re-oxidation. Encapsulation is facilitated by placing the aluminium components in a chamber of an electron beam welding device, evacuating the chamber and electron beam welding the edge regions of the components together. As will be appreciated, this is a costly additional step in the manufacture of components.